As I said, 15-20 years (and it's just a rough guess). It may even be more, since Madam's Foster aging seems to be in the span of 20-25 years. Frankie seems to be just a child, maybe 2-4 years old in the first picture.

I forgot the Rabbit guy's name, but wasn't he Frankie's imaginary friend?
Can imaginary friends make imaginary friends?
My head hurts, it's been forever since I watched this show. Sorry for being stupid.